Method for setting the operation of a routing node of an asynchronous wireless communication network, network node and communication network implementing the method

ABSTRACT

To reduce energy consumption in a duty-cycled asynchronous wireless communication network values of operation parameters, i.e. duration of the awake interval and duration of the sleep interval, of routing nodes of the network are determined and set. The network is partitioned into clusters so that each cluster comprises one cluster-head node. The energy consumption of a cluster is a function of the probability of busy channel when nodes of the cluster attempt transmission, the probability of communication collision during transmission, the duration of the awake interval and the duration of the sleep interval of its cluster-head node. Reduction of the energy consumption is carried out under predetermined values of the probability of busy channel and of the probability of communication collision and under predetermined constraint for the probability of successful transfer of data packets within the cluster and for the average delay of transfer of data packets within the cluster.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Phase application under 35 U.S.C. §371 ofInternational Application No. PCT/EP2008/008925, filed Oct. 22, 2008,which was published Under PCT Article 21(2), the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for setting the operation of arouting node of an asynchronous wireless communication network and to anetwork node and a communication network implementing it.

BACKGROUND OF THE INVENTION

WPAN [Wireless Personal Area Network] networks are known for some years;a PAN [Personal Area Network] network can be defined as a computernetwork for communicating among devices close to one person; a WPANnetwork is a PAN network using wireless short-range communicationtechnologies.

A communication technology which is very often used for implementing aWPAN network is ZigBee.

One of the main and recent applications of WPAN networks is WSN[Wireless Sensor Network] networks.

In a WPAN network the key components are the nodes of the network, alsocalled devices. In general, a WPAN network may comprise a mixture ofmains powered devices and battery powered devices; battery powereddevices are designed to limit their energy consumption so to assure along lifetime to their batteries. Providing efficient use of energy inWSN networks is particularly important in order to achieve long-termdeployment of applications since the sensor network nodes may not beeasily recharged or replaced when the energy of their battery is over.

The component of a node of a WPAN network which is primarily responsiblefor energy consumption is the radio transceiver (both when it transmitsand when it receives); the typical and effective way of reducing energyconsumption in an asynchronous WPAN network (a network wherein the nodesdo not have a synchronized clock and therefore do not transmit andreceive synchronously) is to use “duty-cycling”, i.e. to let the radiotransceiver of the devices operate intermittently for short intervals oftimes; in this way, the operation of each node is a periodic (fixedtransceiver operation period) sequence of a (short) awake interval and a(long) sleep interval. Of course, this complicates the communicationprotocols used in WPAN networks.

From the prior art, there are known asynchronous WSN networks whereinall (or almost all) the nodes are battery powered and therefore aredesigned to limit energy consumption and wherein special MAC protocolsare used to limit energy consumption of the radio transceivers.

The article by J. Polastre et al, “Versatile Low Power Media Access forWireless Sensor Networks”, SenSys 2004, November 2004, describes indetail one of such MAC protocols called “B-MAC” based on “preamblesampling”. According to this protocol, when a sender node has data totransmit, it transmits a preamble that lasts at least as long as thesleep period interval of the receiver node (this duration is referred toas “preamble length”); when the receiver node wakes up (this happensaccording to a period referred to as “check interval”), it detects thepreamble and stays awake to receive data. This article also mentions theuse of a set of bidirectional interfaces that allow an application tochange the “check interval” and the “preamble length” in order tooptimize energy consumption, latency and throughput and adapt tochanging network conditions.

The article by M. Buettner et al, “X-MAC: A Short Preamble MAC Protocolfor Duty-Cycled Wireless Sensor Networks”, SenSys 2006, November 2006,describes in detail another of such MAC protocols called “X-MAC” basedon “preamble sampling” and a sequence of short fixed-length preambles.According to this protocol, when a source node has to transmit aninformation packet, its transmitter transmits a series of short andfixed-length preambles, each containing the address of the destinationnode; small pauses between preambles permit the receiver of thedestination node to awake (according to its own internal operationschedule), to detect the preamble and to send an acknowledgment thatstops the sequence of preambles and signals the availability of thedestination node to receive data; non-destination receivers whichoverhear the strobed preambles can go back to sleep immediately, ratherthan remaining awake for receiving data. This article also describes anadaptive algorithm which can be used to dynamically adjust duty-cycleparameters in order to optimize for energy consumption per packet,latency or both.

A different approach for reducing energy consumption is disclosed byU.S. Pat. No. 7,035,240; this patent deals with a method and networkarchitecture for implementing an energy efficient network. The networkincludes a plurality of nodes that collect and transmit data that areultimately routed to a base station. The network nodes form a set ofclusters with a single node acting as a cluster-head. The cluster-headadvertises for nodes to join its cluster, schedules the collection ofdata within a cluster, and then transmits the data to the base station.A cluster can intelligently combine data from individual nodes. After aperiod of operation, the clusters are reformed with a different set ofnodes acting as cluster-heads. The network provides an increased systemlifetime by balancing the energy use of individual nodes.

SUMMARY OF THE INVENTION

The Applicant has considered that in asynchronous wireless communicationnetworks, especially in WPAN/WSN networks, there is a need for a muchbetter optimization of the energy consumption of network nodes withrespect to known protocols for the following reasons.

The solution known from U.S. Pat. No. 7,035,240 is based on a high-levelapproach that requires load balancing and data aggregation in thecluster-head nodes of the network and provides for a synchronous networkwithin each cluster of the network based on a TDMA [Time DivisionMultiple Access] schedule.

The solutions known from the two above-mentioned articles do not takeinto account the effect of random access which is a function of datatraffic, MAC parameters and network topology and which is responsiblefor a large part of energy consumption; in other words, the energyconsumption model is too simplified and can not lead to any trueminimization.

The present invention aims at improving the solutions of the prior art.

In particular, the present invention addresses the problem of minimizingenergy consumption in a duty-cycled asynchronous wireless communicationnetwork. The basic idea behind the present invention is to use an energyconsumption model that takes into account the effect of random access;anyway, in order to simplify the model, the network is partitioned intoa plurality of clusters so that each of said clusters comprises onecluster-head node; for the purpose of this partitioning, a cluster-headnode is a routing node considered only receiving data packets from theother nodes of the cluster and the other nodes of the cluster are nodesconsidered only transmitting data packets to the cluster-head node—thismeans that according to this partitioning clusters may partiallyoverlap.

In order to build the model, the Applicant has considered that whentransmission within a cluster is attempted by any of the nodes of thecluster a probability of busy channel is encountered, that whentransmission within a cluster is carried out by any of the nodes of thecluster a probability of communication collision is encountered, andthat a data packet is transferred within a cluster with a probability ofsuccessful transfer (which may be called “reliability”) and with anaverage delay of transfer (which may be called “latency”); additionally,the Applicant has considered the influence of the number of nodes of theclusters and of the average data packet generation rate according towhich data packets are transmitted by the nodes of the clusters.According to the herein proposed model, the energy consumption of acluster is a function primarily of the probability of busy channel, theprobability of communication collision, the duration of the awakeinterval and the duration of the sleep interval of the cluster-headnode; additionally, the energy consumption is a function of the numberof nodes of the cluster and the average data packet generation rate.

Minimization of the energy consumption is carried out underpredetermined values of the probability of busy channel and of theprobability of communication collision and under predeterminedconstraint for the probability of successful transfer and for theaverage delay of transfer; in this way, optimal values of the durationof the awake interval and the duration of the sleep interval of thecluster-head node are determined. Once these optimal values aredetermined, they are set and used as operation parameters of thecluster-head node.

The partitioning of the network into a plurality of clusters may befixed and predetermined; alternatively, it may be one of the steps to becarried out according to the present invention only once or repeatedly,for example periodically.

It is to be noted that, in general, the operation parameters of thevarious routing nodes of the network will be different.

The determination of the optimal operation parameters may be repeated,e.g. periodically, during the operation of the network. This isadvantageous for example if the probability of busy channel or theprobability of communication collision or both such probabilities changeduring the operation of the network. These two probabilities may bedetermined by each routing node of the network during its operation; inparticular, the probability of busy channel may be determined byreceiving estimates of the probability of busy channel made by the othernodes of the same cluster and by calculating an average or the maximumof the received estimates.

The optimal values of the operation parameters may be determined by eachrouting node of the network during its operation; in order to do thisquickly and by means of a simple hardware, such determination may bebased on a table of pre-calculated values; such table may be storedinside each routing node; such storage into the routing nodes may becarried out e.g. by a base station; such base station may havecalculated all the values of the table or tables before the networkstarts operating.

According to further aspects, the present invention relates to a networknode and a communication network implementing the above method thatminimizes energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription to be considered in conjunction with the annexed drawing,wherein:

FIG. 1 shows a duty-cycled asynchronous wireless communication networkaccording to the present invention wherein a first cluster and a secondcluster are highlighted,

FIG. 2 shows time diagrams explaining the operation of a source node anda destination node according to an asynchronous duty-cycling MACprotocol based on preamble sampling with multiple short preambles,

FIG. 3 shows a flowchart explaining a method for estimating the busychannel probability according to the present invention,

FIG. 4 shows a flowchart explaining a method for estimating thecollision probability according to the present invention,

FIG. 5 shows a table of values used by a routing node for determiningthe optimal operation parameters according to the present invention, and

FIG. 6 shows a flowchart explaining a method of using the table of FIG.5 by a routing node according to the present invention.

It is to be understood that the following description and the annexeddrawings are not to be interpreted as limitations of the presentinvention but simply as exemplifications.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment of the present invention that will be described in thefollowing relates to a WPAN [Wireless Personal Area Network] network,specifically a WSN [Wireless Sensor Network] network using the ZigBeetechnology, even if the present invention may be applied more in generalto an asynchronous wireless communication network. The operation of anetwork node is timed by means of an own clock signal which is generatedby a timer (usually a local timer) though e.g. a quartz crystal; thetimer is used by the network node also for measuring the duration oftime intervals. As already explained, a network is defined as“asynchronous” if its nodes do not have a synchronized clock andtherefore do not transmit and receive synchronously.

The operation of a duty-cycled asynchronous wireless communicationnetwork is known and is described e.g. in the already-cited article byM. Buettner; additionally, this article describes a MAC protocol basedon the transmission of a sequence of short fixed-length preambles, as itis the case of the embodiment of the present invention that will bedescribed in the following.

FIG. 1 shows a WSN network according to the present invention andembodying a ZigBee mesh topology. This network comprises seven routers,R1, R2, R3, R4, R5, R6, R7, and seven end devices, E1, E2, E3, E4, E5,E6, E7.

FIG. 2 shows time diagrams explaining the operation of a source node SN(for example node E4) and a destination node DN (for example node R3)when data is to be transmitted from the source node SN to thedestination node DN in the network of FIG. 1.

According to a “duty-cycled” transmission scheme, the destination nodeDN wakes up periodically for checking whether there is data for it frome.g. the source node SN; if yes, it receives data; if not, it returns tosleep; this is repeated periodically; the time spent in sleep state iscalled “sleep time” and corresponds to the duration of a sleep intervalR_(s) and the time spent in awake state is called “awake time” andcorresponds to the duration of an awake interval R_(l).

In “preamble sampling” MAC protocols, when the source node has data totransmit to the destination node, it transmits a series of shortpreamble packets, each containing the identifier of the destinationnode, until it either receives an acknowledgement packet from thedestination node or a “time-limit” TL is exceeded which corresponds tothe maximum sleep time of a destination node—in the example of FIG. 2,an acknowledgement packet ACK is received by the source node SN afterthe transmission of only two preamble packets PRE1 and PRE2.

In general, if a destination node wakes up and detects no radio signalsin the communication channel, it returns to sleep immediately; if thedestination node receives a preamble packet, but it is not the target ofthat preamble packet, it returns to sleep after this check; if thedestination node receives a preamble packet and it is the target of thatpreamble packet, it transmits an acknowledgement packet to the sourcenode after reception of the preamble packet. After reception of theacknowledgement packet, the source node transmits a data packet to thedestination node.

The diagrams of FIG. 2 shows further details connected to suchcommunication; it is to be highlighted that the sequence of events inFIG. 2 is only one of the many possibilities that may occur duringoperation of the network of FIG. 1.

A request of transmission RT is sent to the MAC layer from an upperlayer of the source node SN and a random amount of time RB1 (called“random backoff”) is spent before the communication channel is sensed;channel sensing is carried out during a short time interval CS1; at thismoment, there are radio signals in the communication channel, i.e. thechannel is busy (typically is used by another node for transmission),therefore another random amount of time RB2 is spent and then thechannel is sensed again during a time interval CS2; now, the channel isavailable and therefore a preamble packet PRE1 is transmitted by thesource node SN.

After the transmission of the preamble packet PRE1, the source node SNspends a time interval TO1 waiting for a reply from the destination nodeDN; in this case, the time interval TO1 lasts as a “time-out” TO; duringthe time interval TO1, the destination node DN does not reply as thedestination node DN was asleep when the preamble packet PRE1 wastransmitted.

After the time interval TO1, as the destination node DN has not replied,a random amount of time RB3 is spent before the communication channel issensed during a time interval CS3 (the channel is found available) and apreamble packet PRE2 is transmitted by the source node SN; during therandom amount of time RB3, the destination node DN wakes up (timeinstant t_(AW) in FIG. 2).

After the transmission of the preamble packet PRE2, the source node SNspends a time interval TO2 waiting for a reply from the destination nodeDN; in this case, the time interval TO2 lasts less than the “time-out”TO as the destination node DN was awake when the preamble packet PRE2was transmitted and has replied to the source node SN.

After the reception of the preamble packet PRE2, the destination node DNspends a random amount of time RB4 before sensing the communicationchannel during a time interval CS4 (the channel is found available) andtransmitting an acknowledgement packet ACK to the source node SN.

After the reception of the acknowledgement packet ACK, the source nodeSN spends a random amount of time RB5 before sensing the communicationchannel during a time interval CS5 (the channel is found available) andtransmitting a data packet DAT to the destination node DN.

After the transmission of the acknowledgement packet ACK, thedestination node DN simply waits for the data packet DAT and returns tosleep (time instant t_(AS) in FIG. 2) after having received the datapacket DAT.

In “preamble sampling” protocols, the amount of time spent in randomaccess for transmitting a data packet is in general much larger than theamount of time spent in transmission (for example, according to IEEE802.15.4 with default parameters setting, the maximum backoff timebefore data packet transmission is 27.4 ms whereas the transmission timeof a 56 byte data packet is 1.79 ms at 250 kbps); random access dependson data traffic, MAC parameters and network topology; random accesscauses energy consumption; therefore, a precise energy consumption modelshould take into account also random access.

For the energy consumption model according to the present invention, itis assumed that the nodes of the network are organized into clusters;the same model will be applied to each cluster.

In a clustered topology, nodes are organized in clusters with one nodewhich acts as cluster-head node (it is a routing node); all the nodes ofthe clusters (except the cluster-head node) transmit their data directlyto the cluster-head node.

If the network has a “tree” topology, the cluster-head nodes receivedata from all the other nodes of the clusters (that are end devices) andforwards them to a base station that acts as coordinator node; if thenetwork has a “mesh” topology, the cluster-head nodes receives data fromall the other nodes of the clusters and forwards them to router nodes.

The application of the energy consumption model according to the presentinvention to a “tree” topology is straightforward.

In order to apply this model to a “mesh” topology, the network ispartitioned into a plurality of clusters so that each of said clusterscomprises one cluster-head node; for the purpose of this partitioning, acluster-head node is a routing node considered only receiving datapackets from the other nodes of the cluster and the other nodes of thecluster are nodes considered only transmitting data packets to thecluster-head node; in order to take account of the fact that a routingnode may not only receive but also transmit data packets, a routing nodebelongs in general to at least two different clusters but it is thecluster-head node of only one cluster; this means that, according tothis partitioning, clusters may partially overlap.

With reference to FIG. 1, a first cluster CL1 of the network comprisesrouters R2, R3 and R4 and end devices E4 and E5 and considers router R3as its cluster-head node, and a second cluster CL2 of the networkcomprises routers R3, R4, R5 and R6 and no end devices and considersrouter R4 as its cluster-head node; therefore, nodes R3 and R4 belongsto two different clusters, i.e. CL1 and CL2, but when considering thefirst cluster CL1 node R3 only receives data packets and node R4 onlytransmits data packets and when considering the second cluster CL2 nodeR3 only transmits data packets and node R4 only receives data packets.It is to be understood that the complete partitioning of the network ofFIG. 1 provides for seven clusters, one for each router of the network.

Throughout the present description, it is assumed that the data packetgeneration rate of the nodes of the network is such that during a timeduration given by the duration of the sleep interval plus the durationof the awake interval, a node has at maximum one data packet totransmit. Consequently, if a node can not transmit a data packet withinR_(l)+R_(s), then the packet is discarded.

In the following, the acronyms “RX” and “rx” will often be used insteadof “receiver” or “receiving node” and the acronyms “TX” and “tx” willoften be used instead of “transmitter” or “transmitting node”.

All the symbols in the following mathematical expressions are listed andexplained in the below table.

Symbol Meaning TX node a transmitter node RX node a receiver node(cluster head) T₁ random delay spent by the TX node before actuallytransmitting a preamble packet T₂ random delay spent by the TX node fromthe beginning of a transmission until the reception of theacknowledgement T₃ random delay spent by the TX node from the instant ofacknowledgement reception until the transmission of a data packet T_(P)random delay to wait before a data packet is successfully received T_(s)random sleep time of the RX as seen from the TX (it is uniformlydistributed over [0, R_(s)]) T_(s) random listening time of the RX ascomputed upon the reception of a preamble (it is uniformly distributedover [0, R_(l)]) T_(ack) random time before the RX node can access thechannel and send an acknowledgement. T_(TX, out) maximum time that a TXnode waits for an ACK after having sent a preamble. T_(out) maximum timethat a TX node waits from the moment of the reception of an ACK beforegiving up the data packet transmission. N_(P) maximum number ofpreambles that can be sent N_(b) maximum number of back-off to sense thechannel for sending a preamble packet NB_(max) maximum number ofback-offs before declaring a channel access failure N number of nodes ina cluster λ packet generation rate per node d_(TX) probability that a TXnode has a packet to send in the interval R_(s) + R_(l) c probability ofbusy channel b probability of preamble collision p probability of datapacket collision ψ_(min) minimum probability of successful packettransmission (reliability requirement) τ_(max) maximum probability ofmaximum delay (latency requirement) E_(max) maximum energy consumptionper listening-sleeping cycle (energy requirement) S_(p,j) j-th randomback-off time of a preamble μS_(p,j) average of S_(p,j) S_(c) timeduration of channel sensing for clear channel assessment S_(p) timeduration of a preamble packet S_(a) time duration of an acknowledgementpacket S_(d) time duration of a data packet S_(b) time duration offorming the basic time period used by the CSMA/CA algorithm R_(s) sleeptime of the receiver node (cluster head) R_(l) active time of thereceiver node (cluster head) A_(k) event occurring when the channel isbusy for k-1 times B_(k) event occurring when a preamble has to be sentk times before being received in the active time of the RX node and thecorresponding acknowledgement is sent by the RX node and received beforethe time out of the TX node G event that occurs when a preamble issuccessfully received during the active state of the receiver H|G eventthat occurs when the ACK is successfully sent before the time out of theRX expires provided that a preamble is successfully received I|G, Hevent that occurs when the TX sends successfully a data packet providedthat a preamble is successfully received and the ACK is alsosuccessfully received P_(tx) transmit power P_(rx) receive power P_(s)sleep powerModelling of the Delay

The probability of the delay for transferring a data packet from asource node (transmitting node) to a destination node (receiving node orcluster-head node) is given by the following expression

_(Tα)

_(Tl) Pr[(T _(p) ≦t _(max))(T ₃ ≦T _(out))]

D _(max)(R _(l) ,R _(s) ,T _(TX,out) ,T _(out) ,c;t _(max))where E_(Ta) and E_(Tl) denote the statistical average with respect tothe distribution of T_(a) (T subscript A) and T_(l) (T subscript L)respectively; T_(a) (T subscript A) is the random time to wait from thebeginning of the transmissions until the start of the awake interval,T_(l) (T subscript L) is the time duration from the moment in which thepreamble packet is received during the awake interval until the awakeinterval expires, T_(p) is the delay to wait before a data packet issuccessfully received, t_(max) is the maximum delay desired by theapplication, c is the probability of communication collision.

The probability that a packet is delayed of t_(max) or less than t_(max)is given by the following expression

${\Pr\left( {T_{p} \leq t_{\max}} \right)} = {\frac{1}{2}\left( {1 + {{erf}\left( \frac{x - \mu_{T_{p}}}{\sigma_{T_{p}}\sqrt{2}} \right)}} \right)}$

The mean and variance of T_(p) are given by the following expressionsμ_(T) _(p) =μ_(T) ₂ +μ_(T) ₃σ_(T) _(p) ²=σ_(T) ₂ ²+σ_(T) ₃ ²where μ_(T) _(p) and σ_(T) _(p) are the mean and variance of T_(p)respectively, T₂ is the period of time spent by the TX node from themoment when the data packet is internally generated until acorresponding acknowledgement packet reaches the TX node and T₃ is theperiod of time spent by TX node starting just after the acknowledgmentpacket is received and ending just before the data packet istransmitted.

The mean and variance of T₂ are given by the following expressions

$\mu_{T_{2}} = {\sum\limits_{k = 1}^{N_{p}}\;{\left\lbrack {{k\;\mu_{T_{1}}} + \mu_{T_{ack}} + {\left( {k - 1} \right)T_{{TX},{out}}}} \right\rbrack\frac{\Pr\left\lbrack \mathcal{B}_{k} \right\rbrack}{\sum\limits_{k = 1}^{N_{p}}\;{\Pr\left\lbrack \mathcal{B}_{k} \right\rbrack}}}}$$\sigma_{T_{2}}^{2} = {\sum\limits_{k = 1}^{N_{p}}\;{\left( {{k^{2}\sigma_{T_{1}}^{2}} + \sigma_{T_{ack}}^{2}} \right){{\Pr\left\lbrack \mathcal{B}_{k} \right\rbrack}.}}}$wherePr[B _(k)]=(Pr[C _(k) E _(k) ]−Pr[ D _(k)])Pr[T _(ack) ≦T_(TX,out)](1−b)²+(Pr[E _(k) ]−Pr[C _(k) E _(k)])Pr[T _(ack) ≦T_(TX,out)]b(1−b)²+(Pr[E _(k) ]−Pr[C _(k) E _(k)])(1−Pr[T _(ack) ≦T_(TX,out)])Pr[T _(ack) ≦T _(TX,out)](1−b)³+(Pr[E _(k) ]−Pr[C _(k) E_(k)])(Pr[T _(ack) ≦T _(TX,out)])² b(1−b)³,where

$\mspace{20mu}{{{\Pr\left\lbrack {??}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} - {\left( {k - 2} \right)T_{{TX},{out}}}}{k - 1} \right)}},\mspace{20mu}{{\Pr\left\lbrack {\overset{\_}{??}}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} - {\left( {k - 1} \right)T_{{TX},{out}}}}{k} \right)}},\mspace{20mu}{{\Pr\left\lbrack \mathcal{E}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} + T_{l} - {\left( {k - 1} \right)T_{{TX},{out}}}}{k} \right)}},{{\Pr\left\lbrack {{??}_{k}\mathcal{E}_{k}} \right\rbrack} = {{{\Pr\left\lbrack {??}_{k} \right\rbrack}{\Pr\left\lbrack {T_{1} \leq {T_{l} - T_{{TX},{out}}}} \right\rbrack}} + {{\Pr\left\lbrack \mathcal{E}_{k} \right\rbrack}{\left( {1 - {\Pr\left\lbrack {T_{1} \leq {T_{l} - T_{{TX},{out}}}} \right\rbrack}} \right).}}}}}$and

${\Pr\left\lbrack {T_{1} \leq t} \right\rbrack} = {{\frac{1}{2}\left( {1 + {{erf}\left( \frac{x - \mu_{T_{1}}}{\sigma_{T_{1}}\sqrt{2}} \right)}} \right)}\overset{\Delta}{=}{P_{1}(t)}}$

As it appears from the above expressions, the mean and variance of T₂are related to the mean and variance of T₁ (the period of time spent byTX node from the moment when the data packet is internally generateduntil the first corresponding preamble packet is actually transmitted)that are given by the following expressions

${\mu_{T_{1}} = {{{??}\; T_{1}} = {{\sum\limits_{k = 1}^{N_{b}}\;{\mu_{\Sigma_{k}}\frac{\Pr\left\lbrack {??}_{k} \right\rbrack}{\sum\limits_{k = 1}^{Nb}\;{\Pr\left\lbrack {??}_{k} \right\rbrack}}}} = {\sum\limits_{k = 1}^{N_{b}}\;{\mu_{\Sigma_{k}}\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{Nb}\;{c^{k - 1}\left( {1 - c} \right)}}}}}}},{\rho_{T_{1}} = {{{??}\; T_{1}^{2}} = {{\sum\limits_{k = 1}^{N_{b}}\;{\rho_{\Sigma_{k}}{\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{Nb}\;{c^{k - 1}\left( {1 - c} \right)}}.\sigma_{T_{1}}^{2}}}}\overset{\Delta}{=}{\rho_{T_{1}} - \mu_{T_{1}}^{2}}}}}$where

$\mu_{\Sigma_{k}} = {{{??}\left\lbrack \Sigma_{k} \right\rbrack} = {{\sum\limits_{j = 1}^{k}\;{\left\lbrack {\mu_{S_{p,j}} + S_{c}} \right\rbrack.\sigma_{\Sigma_{k}}^{2}}} = {{{??}\left\lbrack {\Sigma_{k} - {??\Sigma}_{k}} \right\rbrack}^{2} = {{\sum\limits_{j = 1}^{k}\;{\sigma_{S_{p,j}}^{2}.\rho_{\Sigma_{k}}}} = {\sigma_{\Sigma_{k}}^{2} + \mu_{\Sigma_{k}}^{2}}}}}}$where

$\mu_{S_{p,j}} = \left\{ {{\begin{matrix}{\frac{\left( {2^{r{(j)}} - 1} \right)S_{b}}{2},} & {{{{for}\mspace{14mu}{r(j)}} = 1},\ldots\mspace{14mu},{{BE}_{\max};}} \\{\frac{\left( {2^{{BE}_{\max}} - 1} \right)S_{b}}{2},} & {{{for}\mspace{14mu}{r(j)}} > {{BE}_{\max}.}}\end{matrix}\sigma_{S_{p,j}}^{2}} = \left\{ {{\begin{matrix}{\frac{\left( {2^{2{r{(j)}}} - 1} \right)S_{b}^{2}}{12},} & {{{{for}\mspace{14mu}{r(j)}} = 1},\ldots\mspace{14mu},{{BE}_{\max};}} \\{\frac{\left( {2^{2{BE}_{\max}} - 1} \right)S_{b}^{2}}{12},} & {{{for}\mspace{14mu}{r(j)}} > {{BE}_{\max}.}}\end{matrix}{r(j)}} = {{rem}\left( {j,{{NB}_{\max} + 1}} \right)}} \right.} \right.$

As it appears from the above expressions, the mean and variance of T₂are related also to the mean and variance of T_(ack) (the period of timefrom the end of the transmission of a preamble packet until the end ofthe transmission of the corresponding acknowledgement packet) that aregiven by the following expressions

${\left. f_{T_{ack}} \right.\sim\frac{1}{\sigma_{T_{ack}}\sqrt{2\pi}}}{\exp\left( {- \frac{\left( {x - \mu_{T_{ack}}} \right)^{2}}{2\sigma_{T_{ack}}^{2}}} \right)}$${\mu_{T_{ack}} = {\sum\limits_{k = 1}^{N_{a}}\;{\mu_{\Sigma_{k}}\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{N_{a}}\;{c^{k - 1}\left( {1 - c} \right)}}}}},{\rho_{T_{ack}} = {\sum\limits_{k = 1}^{N_{a}}\;{\rho_{\Sigma_{k}}\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{N_{a}}\;{c^{k - 1}\left( {1 - c} \right)}}}}},{\sigma_{T_{ack}}^{2}\overset{\Delta}{=}{\rho_{T_{ack}} - {\mu_{T_{ack}}^{2}.}}}$

The mean and variance of T₃ is given by the following expressions

${\mu_{T_{3}} = {\sum\limits_{k = 1}^{N_{d}}\;{\mu_{\Sigma_{k}}\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{N_{a}}\;{c^{k - 1}\left( {1 - c} \right)}}}}},{\rho_{T_{3}} = {\sum\limits_{k = 1}^{N_{d}}\;{\rho_{\Sigma_{k}}\frac{c^{k - 1}\left( {1 - c} \right)}{\sum\limits_{k = 1}^{N_{a}}\;{c^{k - 1}\left( {1 - c} \right)}}}}},{\sigma_{T_{3}}^{2}\overset{\Delta}{=}{\rho_{T_{3}} - {\mu_{T_{3}}^{2}.}}}$Modelling of the Reliability

The probability to transfer a data packet successfully from a sourcenode (transmitting node) to a destination node (receiving node orcluster-head node), which may be called “reliability”, is given by thefollowing expressionR _(min)(R _(l) ,R _(s) ,T _(TX,out) ,T _(out) ,c,p)=

_(Tα)

_(Tl) Pr[

]Pr[I|

].where the probability that a preamble packet is successfully transmittedduring the awake interval of the RX node and the correspondingacknowledgement packet is successfully transmitted before the timeout ofthe TX node expires is given by the following expression

${\Pr\lbrack{??}\rbrack} = {\sum\limits_{k = 1}^{N_{p}}\;{{\Pr\left\lbrack \mathcal{B}_{k} \right\rbrack}.}}$the probability that the TX node transmits a data packet successfully,given that a preamble packet is successfully received and theacknowledgement packet is also successfully received, is given by thefollowing expressionPr[

|

]=(1−c ^(NB) ^(max) )(1−c ^(N) ^(a) )(1−c ^(N) ^(d) )(1−p).Modelling of the Energy Consumption

The average energy consumption (in other words the expected value

of the total energy E_(tot)), normalized by R_(l)+R_(s), for eachcluster of the network is given by the following expression:

E _(tot)=(Nd _(TX)

E _(tx) +

E _(rx))/R _(s) +R _(l))where N is the number of transmitting nodes in the cluster (equal to thetotal number of the nodes of the cluster minus one, i.e. thecluster-head node) and d_(TX) is the probability that a transmittingnode has at least one data packet to transmit to the cluster-head nodeduring the time interval of R_(l)+R_(s), i.e. d_(TX)=1−e^(−λ(R) ^(s)^(+R) ^(t) ⁾, where λ is the average data package generation rate.

In this expression, the first addend is the average energy consumptionof the transmitting nodes and the second addend is the average energyconsumption of the receiving node, i.e. the cluster-head node.

The average energy consumption of one transmitting node is given by thefollowing expression

${{??}\; E_{tx}} = {{\sum\limits_{i = 1}^{N_{p}}\;{\left\{ {{i\;{{??}\left\lbrack E_{{tx},T_{1}} \right\rbrack}} + {\left( {i - 1} \right){??}\; E_{T_{{TX},{out}}}} + {{??}\left\lbrack E_{{tx},T_{ack}} \right\rbrack} + {{??}\; E_{{tx},T_{data}}}} \right\}{\Pr\left\lbrack \mathcal{B}_{i} \right\rbrack}}} + {\left( {{N_{p}E_{{tx},T_{1}}} + {\left( {N_{p} - 1} \right)E_{T_{{TX},{out}}}} + E_{{tx},T_{ack}}} \right){\left( {1 - {\sum\limits_{i = 1}^{N_{p}}\;{\Pr\left\lbrack \mathcal{B}_{i} \right\rbrack}}} \right).}}}$

The first line in the above equation corresponds to the energy spent inthe case of successful data packet transmission and the second linecorresponds to the energy spent in the case of unsuccessful data packettransmission. The four addends in the first line of the above expressionare, respectively, the energy for accessing the channel and transmitting“i” preamble packets given that the event B₁, occurs, namely the i-thpreamble packet was successfully transmitted and the correspondingacknowledgment packet was successfully received, the energy spent during“i−1” (fixed) time out periods of the TX node for the preamble packets,the energy for receiving an acknowledgement packet, the energy fortransmitting a data packet.

The three addends in the second line of the above expression are,respectively, the energy spent when N_(p) (the maximum number ofpreamble packets) preamble packets are transmitted but no data packet istransmitted since either the channel is busy or the awake interval ofthe receiving node has been missed due to collisions or random backoff.

These addends are defined by the following expressions

$\mspace{20mu}{{{{??}\; E_{{tx},T_{1}}} = {\sum\limits_{j = 1}^{N_{b}}\;{\left\lbrack {{\sum\limits_{k = 1}^{j}\;\left( {{P_{s}\mu_{S_{p,k}}} + {P_{rx}S_{c}}} \right)} + {P_{tx}S_{p}}} \right\rbrack{\Pr\left\lbrack {??}_{j} \right\rbrack}}}},\mspace{20mu}{E_{T_{{TX},{out}}} = {T_{{TX},{out}}P_{rx}}},\mspace{20mu}{{{??}\; E_{{tx},T_{ack}}} = {\sum\limits_{j = 1}^{N_{a}}\;{\left\lbrack {{\sum\limits_{k = 1}^{j}\;\left( {\mu_{S_{p,k}} + S_{c}} \right)} + S_{a}} \right\rbrack P_{rx}{\Pr\left\lbrack {??}_{j} \right\rbrack}}}},{{{??}\; E_{{tx},T_{data}}} = {{\sum\limits_{j = 1}^{N_{d}}\;{\left\lbrack {{\sum\limits_{k = 1}^{j}\;\left( {{P_{s}\mu_{S_{p,k}}} + {P_{rx}S_{c}}} \right)} + {P_{tx}S_{d}}} \right\rbrack{\Pr\left\lbrack {??}_{j} \right\rbrack}}} + {\quad{{\left\lbrack {{\sum\limits_{k = 1}^{N_{d}}\;\left( {{P_{s}\mu_{S_{p,k}}} + {P_{rx}S_{c}}} \right)} + {P_{tx}S_{d}}} \right\rbrack{\Pr\left\lbrack \overset{\_}{??} \right\rbrack}},}}}}}$

In these equations, the following terms have been used:Pr[B _(k)]=(Pr[C _(k) E _(k) ]−Pr[ D _(k)])Pr[T _(ack) ≦T_(TX,out)](1−b)²+(Pr[E _(k) ]−Pr[C _(k) E _(k)])Pr[T _(ack) ≦T_(TX,out)]b(1−b)²+(Pr[E _(k) ]−Pr[C _(k) E _(k)])(1−Pr[T _(ack) ≦T_(TX,out)])Pr[T _(ack) ≦T _(TX,out)](1−b)³+(Pr[E _(k) ]−Pr[C _(k) E_(k)])(Pr[T _(ack) ≦T _(TX,out)])² b(1−b)³,where

$\mspace{20mu}{{{\Pr\left\lbrack {??}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} - {\left( {k - 2} \right)T_{{TX},{out}}}}{k - 1} \right)}},\mspace{20mu}{{\Pr\left\lbrack {\overset{\_}{??}}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} - {\left( {k - 1} \right)T_{{TX},{out}}}}{k} \right)}},\mspace{20mu}{{\Pr\left\lbrack \mathcal{E}_{k} \right\rbrack} = {P_{1}\left( \frac{T_{a} + T_{l} - {\left( {k - 1} \right)T_{{TX},{out}}}}{k} \right)}},{{\Pr\left\lbrack {{??}_{k}\mathcal{E}_{k}} \right\rbrack} = {{{\Pr\left\lbrack {??}_{k} \right\rbrack}{\Pr\left\lbrack {T_{1} \leq {T_{l} - T_{{TX},{out}}}} \right\rbrack}} + {{\Pr\left\lbrack \mathcal{E}_{k} \right\rbrack}{\left( {1 - {\Pr\left\lbrack {T_{1} \leq {T_{l} - T_{{TX},{out}}}} \right\rbrack}} \right).}}}}}$

The energy consumption at the cluster-head node (“E_(rx)”) is given bythe energy spent for idle listening, sending an acknowledgement packetand receiving a data packet. It is difficult to provide a closed formexpression for the probabilities of these events, because they arehighly cross-correlated among them and among different nodes. As aresult, an accurate characterization would require modelling theprobability that the receiving node is busy with the reception of a datapacket while some other node is trying to send another data packet.Hence, by considering that the energy spent at the receiving node issmall if compared to the energy spent at the transmitting node, thefollowing upper bound is used:

E _(rx) ≦R _(s) P _(s)+(R _(l) +T _(out))max(P _(tx) ,P _(rx)).where it is assumed that the receiving node can be listening for atime-out (“T_(out)”=fixed time out periods of the RX node for the datapacket), after the end of the awake interval if an acknowledgementpacket was transmitted just before the end of the awake interval.Minimization of the Energy Consumption

Before using the expressions of the energy, delay and reliability justdetermined, it is necessary to determine the probability of busy channelwhen attempting transmission and the probability of communicationcollision during transmission. Afterwards, optimal values of theduration of the awake interval and the duration of the sleep intervalthat reduce under a set threshold, for example minimize, the energyconsumption may be determined.

According to the best embodiment of the present invention, both theprobability of busy channel and the probability of communication arevariable and are repeatedly (for example periodically) determined foreach cluster during operation of the network and their values is thenused for repeatedly determining optimal values of the duration of theawake interval and the duration of the sleep interval for eachcluster-head node.

Determining Busy Channel Probability

Busy channel probability is estimated at each non-cluster head nodewhile attempting transmission of a packet (a preamble packet or datapacket) to the cluster-head node. FIG. 3 shows a possible flowchart ofthe method for estimating the busy channel probability.

The estimate made by a node may be transmitted to the cluster-head nodein the payload of a preamble packet; preferably, every time a preamblepacket is transmitted to the cluster-head node, the transmitting nodeinserts into the payload its last estimate of the busy channelprobability.

The cluster-head node may store the busy channel probability estimatesreceived from its neighbouring nodes (for example one value for eachneighbouring node) into a table; the busy channel probability to be usedfor the determination of the optimal operation parameters is preferablycalculated as the average of the values stored in this table;alternatively, for example, the busy channel probability to be used forthe determination of the optimal operation parameters may be calculatedas the maximum of the values stored in this table.

The example of FIG. 3 refers to the case of unslotted IEEE 802.15.4CSMA/CA [Carrier Sense Multiple Access with Collision Avoidance]mechanism: each node in the network has two variables: NB and BE. NB isthe Number of Backoffs (that is a unit period of time for measuring arandom period of time, which is called “random backoff time”) used whileattempting the current transmission; NB is initialized to 0 before everynew transmission. BE is the Backoff Exponent, which is related to howmany backoffs a node must wait before it attempts to assess the channel.The parameters that affect the random backoff time are minBE, maxBE andmaxNB, which correspond to the minimum and maximum of BE and the maximumof NB respectively. NB is initialized to 0 and BE is initialized tominBE (step 301). The MAC layer delays for a random number of backoffsin the range from 0 to 2^(BE)−1 (step 1 302) and then requests thephysical layer to perform a CCA [Clear Channel Assessment] (step 303).The result of the CCA is considered (step 304): if the channel is busy(arrow N outgoing from block 304), the channel is not accessed and theMAC layer increments both NB and BE by one, ensuring that BE remainsless than maxBE (step 306), if the channel is available (arrow Youtgoing from block 304) the channel is accessed and the method endssuccessfully (step 310). After incrementing NB and BE, a check isperformed on the value of NB (step 307), namely whether the value of NBis more than maxNB; if the check is positive (arrow Y outgoing fromblock 307), the channel is not accessed and the method endsunsuccessfully (step 308); if the check is negative (arrow N outgoingfrom block 307), the MAC layer delays for a random number of backoffsusing the updated values of NB and BE (step 302). The busy channelprobability “c” is initialised to an initial value “c₀” and is updatedduring the above process: if the channel is busy (arrow N outgoing fromblock 304) it is updated to c=αc+(1−α)1 (step 305) and if the channel isavailable (arrow Y outgoing from block 304) its is updated to c=αc (step309), wherein α is a weighting factor, for example 0.9 or 0.99.

Determining Collision Probability

A transmitting node might conclude that a collision has occurred to atransmitted preamble packet when no acknowledgement packet is receivedin reply thereto; anyway, the reason for the failure to get back anacknowledgement packet is, in most cases, that the destination node issleeping during the transmission.

Therefore, according to the present invention, the collision probabilityis advantageously determined at the receiver (i.e. the cluster-headnode) side, which is the cluster-head node. FIG. 4 shows a possibleflowchart of the method for determining the collision probability at anyreception attempt.

A counter “n” is initialized to 0 (step 401). The cluster-head nodechecks the RSSI [Received Signal Strength Indicator] (step 402). If theRSSI is more than the receiving threshold “R_(th)” (arrow Y outgoingfrom block 402), packets are being transmitted; otherwise (arrow Noutgoing from block 402) packets are not being transmitted and thecounter is reset (step 401). If there is transmission, the cluster-headnode counts the number of symbols during which the RSSI is above thereceiving threshold by incrementing counter “n” (step 403) and comparingits value with minimum number of symbols forming a packet “N_(th)” (step404). If the number of symbols so counted is greater than N_(th), (arrowY outgoing from block 404) a packet is being received and a check iscarried out on the preamble bits (step 405). If the preamble bits arenot successfully detected (arrow N outgoing from block 405) thecluster-head node assumes there has been a collision and updates thecollision probability to p=βp+(1−β) 1 (step 406). If the preamble bitsare successfully detected (arrow Y outgoing from block 405) the CRC[Cyclic Redundancy Check] is checked (step 407). If CRC does not check(arrow N outgoing from block 407) there has been a collision and thecollision probability is updated to p=βp+(1−β)1 (step 406), if it doescheck (arrow Y outgoing from block 407) the collision probability isupdated to p=βp (step 408), wherein β is a weighting factor, for example0.9 or 0.99.

Optimization Problem

Given the expressions of the energy, delay and reliability as functionof the durations of the sleep and awake intervals of the cluster-head,number of nodes, average data packet generation rate, busy channel andcollision probability, the energy is minimized subject to reliabilityand delay constraints as follows:

${??}\text{:}\mspace{31mu}{\min\limits_{{R_{l},R_{s},{T_{{TX},{out},}T_{out}}}\;}\;{E\left( {R_{l},R_{s},T_{{TX},{out}},T_{out},c,p,\lambda} \right)}}$        s.t.     D_(max)(R_(l), R_(s), T_(TX, out), T_(out), c, t_(max)) ≤ τ_(max),              R_(min)(R_(l), R_(s), T_(TX, out), T_(out), c, p)≥ ψ_(min),where τ_(max) is the desired probability that the delay is less thanτ_(max) and ψ_(min) is the minimum desired probability with which a datapacket should be transferred. In this optimization, c and p arefeed-forward variables whereas λ, t_(max), τ_(max) and ψ_(min) areapplication requirements.

The solution of this minimization problem with respect to the operationparameters R_(s) and R_(l) can be obtained via standard numericaltechniques. The person skilled in the art may determine, in eachspecific situation, values of the durations of the sleep and awakeintervals of the cluster-head, number of nodes, average data packetgeneration rate, busy channel and collision probability that, subject toreliability and delay constraints, achieve energy levels that are belowa set threshold, such threshold level being sufficiently close, from apractical point of view, to the minimum energy level. These values ofthe durations of the sleep and awake intervals of the cluster-head andof the other parameters indicated above that bring the energy levelbelow the set threshold will be also considered as fulfilling theminimum energy condition.

According to an advantageous embodiment of the present invention, thisminimization problem is solved offline, by means of e.g. a base stationconnected to the network, for many representative values of fivevariables, namely the total packet generation rate in the cluster (i.e.the number of nodes multiplied by the average data packet generationrate), collision and busy channel probabilities, delay and reliabilityconstraints; in this way, the nodes of the network do not need a complexhardware for carrying out complicated calculations in real-time.

The values of the five parameters as well as the correspondingpre-calculated optimal values R_(l) ^(opt) and R_(s) ^(opt) of theoperation parameters R_(l) and R_(s) are stored into a table TBL likethe one shown in FIG. 5 comprised in each of the cluster-head nodes ofthe network.

The method of using the table TBL of FIG. 5 (or a similar table) will bedescribed with the help of the flowchart of FIG. 6 in relation to thecluster-head node of one of the clusters of the network.

At the initialization of the cluster-head node (that can be carried outby e.g. a base station) the values of three variables are stored intothe cluster-head node: the total packet generation rate Nλ of itscluster, the desired probability τ_(max) that the delay is less thant_(max), the minimum desired probability ψ_(min) with which a datapacket should be transferred; additionally, the operation parameters ofthe cluster-head node are set to e.g. default values. The cluster-headnode receives from its neighbouring nodes the estimates of the busychannel probability and calculates their average (step 601); the clusterheads estimates the collision probability (step 602); using as entrypoints the values of the three variables, the calculated average busychannel probability and the estimated collision probability, two valuesof R_(l) and R_(s) are read from the table TBL corresponding to theoptimal vales R_(l) ^(opt) and R_(s) ^(opt) that minimizes the energyconsumption of the cluster (step 603) and are set as new operationparameters of the cluster-head node replacing the previous operationparameters (step 604); after that, the cluster-head node waits some time(step 605) before repeating the steps of the method.

1. Method for setting operation of a routing node of an asynchronouswireless communication network, wherein said network includes aplurality of clusters, each of said clusters having one cluster-headnode, wherein a cluster-head node is a routing node receiving datapackets transmitted from other nodes of the cluster, the methodcomprising the steps of: A) for a cluster wherein said routing node isthe cluster-head node, determining energy consumption, by thecluster-head node, as a function of parameters including probability ofbusy channel, probability of communication collision, probability ofsuccessfully transfer and average delay, B) for the cluster wherein saidrouting node is the cluster-head node, determining an awake interval anda sleep interval of said routing node as a function of the determinedenergy consumption by comparing the function of the determined energyconsumption to a set of predetermined thresholds associated withpredetermined values for probability channel being busy , probability ofcollision, probability of successful transfer, and average delay whichis associated with a minimum energy consumption value, and C) settingthe awake interval and the sleep interval of said routing node forprocessing data packets received from other nodes in the cluster basedon the awake interval and the sleep interval determined in step B). 2.Method according to claim 1, further comprising the step of partitioningsaid network into the plurality of clusters each including onecluster-head node.
 3. Method according to claim 1, wherein thedetermining in step A) includes determining the energy consumption as afunction of the parameters further including a number of nodes in thecluster other than the cluster-head node and an average data packetgeneration rate at which the number of nodes in the cluster transmitdata packets.
 4. Method according to claim 1, further comprisingperiodically determining, by the cluster-head node, the probability ofbusy channel.
 5. Method according to claim 4, wherein the determining ofthe probability of busy channel includes receiving estimates of theprobability of busy channel made by the other nodes of the cluster andcalculating an average or maximum of the received estimates.
 6. Methodaccording to claim 1, further comprising periodically determining, bythe cluster-head node, the probability of communication collision bymaking estimates.
 7. Method according to claim 1, wherein thedetermining in step B) includes retrieving the set of predeterminedthresholds associated with the predetermined values from a table ofvalues stored in the cluster-head node.
 8. Node of an asynchronouswireless communication network including a plurality of clusters, thenode being in a cluster with other nodes, the node comprising: a radiotransceiver having an intermittent operation, the intermittent operationcorresponding to a periodic sequence of an awake interval and a sleepinterval, said radio transceiver being used at least for receiving datapackets, and a processor configured to set the operation of the radiotransceiver by determining energy consumption as a function ofparameters including probability of busy channel, probability ofcommunication collision, probability of successfully transfer andaverage delay, determining the awake interval and the sleep interval asa function of the determined energy consumption by comparing thefunction of the determined energy consumption to a set of predeterminedthresholds associated with predetermined values for probability channelbeing busy, probability of collision, probability of successfultransfer, and average delay which is associated with a minimum energyconsumption value, and setting the awake interval and the sleep intervalfor processing data packets received from other nodes in the cluster tothe determined awake interval and the determined sleep interval. 9.Asynchronous wireless communication network comprising: one or morerouting nodes, each of the one or more routing nodes including radiotransceiver configured to operate intermittently, intermittent operationcorresponding to a periodic sequence of an awake interval and a sleepinterval, said radio transceiver being used at least for receiving datapackets, and a processor configured to set the operation of the radiotransceiver by determining energy consumption as a function ofparameters including probability of busy channel, probability ofcommunication collision, probability of successfully transfer andaverage delay, determining the awake interval and the sleep interval asa function of the determined energy consumption by comparing thefunction of the determined energy consumption to a set of predeterminedthresholds associated with predetermined values for probability channelbeing busy, probability of collision, probability of successfultransfer, and average delay which is associated with a minimum energyconsumption value, and setting the awake interval and the sleep intervalfor processing data packets received from other nodes in the cluster tothe determined awake interval and the determined sleep interval. 10.Method for assigning an awake interval and a sleep interval toprocessing performed by a cluster head node, said method comprising:determining energy consumption, by a cluster head node, as a function ofparameters including probability channel is busy, probability ofcollision, probability of successful transfer, and average delay,wherein the parameters are associated with the channel between thecluster head node and network nodes in the same cluster as the clusterhead node; determining the awake interval and the sleep interval as afunction of the determined energy consumption by comparing the functionof determined energy consumption to a set of predetermined thresholdsassociated with predetermined values for probability channel being busy,probability of collision, probability of successful transfer, andaverage delay which is associated with a minimum energy consumptionvalue; and assign the awake interval and the sleep interval associatedwith the minimum energy consumption value to the cluster head node forprocessing data received from the cluster network nodes.
 11. Methodaccording to claim 10, wherein the determining the energy consumptionincludes determining the energy consumption as a function of theparameters further including a number of the cluster network nodes andan average data packet generation rate at which the number of thecluster network nodes transmit data packets.
 12. Method according toclaim 11, further comprising periodically determining, by thecluster-head node, the probability of busy channel).
 13. Methodaccording to claim 12, wherein the determining of the probability ofbusy channel includes receiving estimates of the probability of busychannel made by the cluster network nodes and calculating an average ormaximum of the received estimates.
 14. Method according to claim 10,further comprising periodically determining, by the cluster-head node,the probability of communication collision by making estimates. 15.Method according to claim 10, wherein the determining of the awakeinterval and the sleep interval includes retrieving the set ofpredetermined thresholds associated with the predetermined values from atable of values stored in the cluster-head node.